Bi-directional actuation of a device between predetermined travel limits, as for example the opening and closing of a door or a gate, has been accomplished in the prior art by means of the single electric motor. This has been believed in the past by those skilled in the art to be the best and simplest means for accomplishing the actuation since an electric motor is readily reversible by reversing the connections of one of its windings to the power source while maintaining intact the connection of the other winding or windings.
Also, it is customary to provide a speed reducing transmission means disposed between the motor output shaft and the input shaft or actuating arm of the device to be actuated. Furthermore, there is usually a travel limit switch arrangement coupled to the transmission mechanism in order to provide a positive means for opening the circuit between the electrical power source and the motor at end limits of travel.
However, as can be seen, the requirement for reversal of motor windings can only be met by bringing out winding leads for the winding to be reversed, as well as the usual leads to the other winding or windings. Accordingly, the solenoid switches or relays which are usually provided for coupling the windings to the power source should be mounted in the immediate vicinity of the motor in order to reduce lead length as much as possible since there are always at least three leads from the switches to the motor which carry current during motor operation.
The prior art practice, as thus outlined briefly, has been found to be undesirable in at least two aspects. Firstly, there is a complexity of electrical power and control circuits which results in a high degree of unreliability. Secondly, the arrangement may require an overdesign of components, and especially so if repeated duty cycles of operation are required at closely spaced intervals. That is, if the actuation is that of a garage door, for example, which is subject to heavy in-and-out traffic over prolonged periods of time, the single motor of the actuator is operating constantly, or very nearly constantly on a near 100% duty cycle. In such a case the motor may heat up and acquire a heat soak if its horsepower or temperature rating is marginal to any extent. Thus, overdesign will call for a more expensive motor having at least a higher temperature rating, or in the alternative, a motor of higher horsepower.
Complexity of control circuit enters into the picture because of the winding reversal switching that is necessary. Furthermore, if the power switch means must be mounted remotely from the motor, the additional motor winding leads must be brought out of the motor and run to the remote switch box.
Thus, a single reversible electric motor for bidirectional operation presents problems of reliability of the motor, the leads and the switching circuit, all of which may be complicated by a further problem imposed by limit switches which may have to be bypassed by control switches when motor reversal is to be effected.